Provide a new cmake rule to compile openvpn3 as a static library

[YungRaj]

Before I submit a pull request I want to hear people's thoughts on this.

[dimonpanik]

that's a good suggestion. in my job I could use a library. it's easier to update the library than the whole application

[schwabe]

I don't think this will work without major refactoring. Currently OpenVPN3 is setup to be in part as header only project. You will face singnificant challenges doing this.

[YungRaj]

Yeah I realized this after creating the bug once I made changes that would compile the library as a static library. This turned out to be a pain in the arse because since the openvpn3 is a header only library, including only 1 or 2 headers from it pulls in pretty much the whole library into the compilation. This causes issues with the linker complaining about duplicate symbols.

I recommend cloning this library into your project and then including the headers yourself so that it eventually gets compiled to the same effect as using this as static library.